feat(completion): support PowerShell

[phucnguyen1707]

Summary

• add native PowerShell completion generated from the same command, engine, tool, MCP, and skill model as the existing shells
• cover top-level commands, install/uninstall/upgrade targets, nested verbs, and supported flags
• accept both powershell and pwsh as generator names, with PowerShell added to CLI help and setup docs
• validate the generated script and real completion behavior with Windows PowerShell 5.1 TabExpansion2

Tests

• node --test --test-concurrency=1 test/completion.test.mjs test/completion-powershell.test.mjs test/completion-uninstall.test.mjs — 28 passed, 2 unavailable-shell skips
• repository-wide suite excluding the three Windows-incompatible baseline files — 606 passed, 192 dependency/platform skips
• the complete upstream suite's 6 Windows shell/process failures were reproduced unchanged on clean upstream f268387; no completion-related failure
• npm pack --dry-run — passed